Thus, these mutants can only be maintained as heterozygotes. (1) Female- or male-specific gametophytic mutants that affect the development or function of the gametophytes: If they are fully penetrant, such mutant alleles can only be transmitted via the other, nonaffected gametophyte. In these screens, three classes of gametophytic mutants were categorized according to the transmission efficiency of mutant alleles and their effects on developmental stages. The genetic principle of these screens is based on the distorted inheritance of mutant alleles that affect gametophytic functions, which does not follow the classical Mendelian rules of segregation typical of mutations affecting sporophytic functions ( Moore et al., 1997 Howden et al., 1998).
![zix file extractor online zix file extractor online](https://www.techlug.com/wp-content/uploads/2019/08/extract-RAR-files-on-Mac.jpg)
In particular, several large-scale screens for Ds transposon or T-DNA insertion mutants defective in Arabidopsis thaliana embryo sac development and other reproductive functions have been performed to identify gametophytic maternal effect ( GME) mutants ( Christensen et al., 1998 Grossniklaus and Schneitz, 1998 Howden et al., 1998 Moore, 2002 reviewed in Brukhin et al., 2005 Pagnussat et al., 2005). Since developmental progression of the fertilization products occurs within female tissues of both gametophytic and sporophytic origin, the extent of maternal contributions to embryo and endosperm growth and development has been a research subject of intense interest. The two male gametes are contained inside the male gametophyte or pollen grain.
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The two female gametes are harbored within the female gametophyte or embryo sac. During double fertilization, the male gametes (two haploid sperm cells) unite with the female gametes (the haploid egg cell and the diploid central cell) to form the embryo and endosperm, respectively. The gametes are formed by the haploid gametophytes, which develop inside the diploid sporophytic reproductive organs of the flowers. The embryo and the endosperm of sexually reproducing flowering plants are derived from male and female gametes and thus carry genomes of different parental origin. Collectively, our results reveal a reproductive function of plant Arm proteins in promoting early seed growth, which is achieved through a distinct GME of ZIX that involves mechanisms for maternal allele-specific expression that are independent of the well-established pathways. The maternal ZIX allele is required for the maternal expression of MINISEED3. The ZIX protein is localized in the cytoplasm and nucleus of cells in reproductive tissues and actively dividing root zones. This parent-of-origin–dependent expression is regulated by neither the histone and DNA methylation nor the DNA demethylation pathways known to regulate some other GME mutants. Expression studies revealed that ZIX manifests a GME through preferential maternal expression in the early embryo and endosperm. ZIX encodes an Armadillo repeat (Arm) protein highly conserved across eukaryotes. We present a detailed analysis of the GME mutant zak ixik ( zix), which displays delayed and arrested growth at the earliest stages of embryo and endosperm development.
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The characterization of many gametophytic maternal effect ( GME) mutants affecting seed development indicates that there are certain classes of genes with a predominant maternal contribution. The proper balance of parental genomic contributions to the fertilized embryo and endosperm is essential for their normal growth and development.